The reproductive epidemiology program emphasizes the development and application of new methods for measuring and analyzing human reproductive outcomes. Such outcomes include fertility, sub-clinical early fetal loss, spontaneous abortion, fetal growth, and birthweight. Each of these outcomes can be affected by environmental factors, and represents a possible endpoint for studying the effects of toxins on human reproduction. One major component of this program is a prospective study of early fetal loss among 200 women. Daily urine specimens are being collected from women who have discontinued their use of birth control in order to become pregnant. Urine assays for human chorionic gonadotropin are being used to estimate the risk of early pregnancy loss among these women. A pilot study of the first thirty women enrolled found chemically evidence of four sub-clinical pregnancy losses. Risk of early loss will be studied in relation to common exposures in this population, such as use of alcohol, tobacco, caffeine beverages and medications. Another area of interest is the possible usefulness of measuring fertility through retrospective estimates of time to pregnancy. A pilot study using this approach has recently been completed. Another pilot study is in progress that looks for sub-clinical conceptions among women using IUD's as possible measure of human fertility. Work continues on the further development of a new method for the analysis of birth weight.